I created a prototype time-lapse camera for a 2 year building site and like to share it here, I learned a lot from forums like this one and hope to contribute a bit showing my project.
For the switch I used Arduino and FONA 808 to turn on the camera only at daytime (LDR) and send me notifications of the battery voltage and whether the camera is turned on or off. The system works on a 12V Lead Acid Gel (deep discharge) battery and solar panel. The Arduino switches a latching relay which only takes current when being switched, and a regular relay which turns on another Arduino with a FONA for 2 minutes to send a text message.
The coding is as simple as I could make it, to make the arduinos as reliable as possible. I included a watchdog timer in the switching Arduino to restart it if it would crash. This Arduino is a Diecimila with a Atmega328 UNO chip (because the Watchdog wouldn’t work on my 168 chip, the boot loader is too slow).
For both Arduino’s I also used the Sleep functions with the JeeLib library to reduce the current as much as possible.
The circuit is fed with a solar charge controller which you can get pretty cheap these days, you just attach a 12V battery (mine is 20Ah) and the solar panel.
In the circuit design you can see a 7812 voltage regulator, I only had it to test it with a 16V power adapter. For 12V batteries you don’t need it. The Arduino Diecimila is fed with 12V but it handles is nicely (it just converts it on the board). The UNO however needs 5V to do good volt metering, it uses it’s internal reference to recalculate the actual voltage and then meters the battery voltage.
Later I removed the 7805 voltage regulator as well and used a buck converter to power the UNO + FONA, it’s much more efficient and saved me 70mA.
My aim is to develop this into a fully reliable system, also with more capabilities.